Photodynamic therapy (PDT) induces a rapid cellular death used to monitor the effect of the treatment immediately following light delivery. The cellular death causes a reduction in NADH fluorescence, which can provide a reliable probe of whether the tissue has had sufficient dose to induce tissue death. Dosimetry based upon measurement of cellular death via NADH fluorescence is a direct measure of the tissue function and can therefore replace more complex methods of dosimetry in vivo. This concept is especially important in human tumors where PDT is being used in complex geometries, such as when applied after surgical debulking of the tumor. In addition, the ability to monitor fluorescence quantitatively from tissue can be accomplished through microsampling of the tissue, using fiberoptics which are smaller than the average scattering length of tissue. Fiber probes smaller than 100 microns can be used to measure both NADH fluorescence as well as photosensitizer fluorescence, and the signal is linearly correlated to the concentration in most tissues. The goals of this study are to develop a functional dosimetry system that has multiple fiber probes, which can be implanted within or on the margins of the tissue to be treated. Measurement of the photosensitizer concentration prior to treatment, as well as the NADH loss in response to treatment provides a complete set of parameters to accurately track the dosimetry in vivo. This system will be calibrated in tissue simulating media, as well as in tumor cell suspensions. The system will be evaluated in the RIF-1 murine tumor model for its ability to predict tumor necrosis. This project employs two innovative observations developed in preliminary studies, including (i) the microsampling technique for quantifying fluorescence of tissue in vivo and (ii) NADH monitoring of cell death in response to PDT. This is the first approach to developing an intrinsic cell metabolism dosimetry system that can be applied in practical situations. This type of system may eliminate the need to monitor many more difficult parameters such as photobleaching, tissue oxygen, blood flow, etc, and therefore could simplify PDT dosimetry significantly. [unreadable] [unreadable] [unreadable]